The present invention relates generally to infrared detectors and, more specifically, to a thermopile infrared detector having an integrated light concentrator to increase the amount of infrared power sensed by the detector.
Non-contact temperature measurement may be accomplished using a conventional infrared detector. Such infrared detectors are suitable for a variety of applications, including HVAC control systems for automotive applications. In this application, infrared detectors are directed toward the driver and passenger in the vehicle. The detectors sense the infrared power emitted from the occupants"" skin, clothing, and surrounding portions of the vehicle interior, and convert that power to heat. Thermocouples in the detector convert the heat flux to a corresponding sensor output voltage which represents the temperature of the object.
More specifically, conventional thermopile detectors include a silicon frame which defines an opening. A thermally isolating membrane spans the opening. An absorber region is created on the membrane and is centered in the opening. The portion of the membrane between the outer dimensions of the absorber region and the inner dimensions of the opening thermally isolates the absorber region from the frame.
A plurality of thermocouples are connected in series and extend across this thermally isolating portion of the membrane between the frame and the absorber. As incident infrared light reaches the absorber, the infrared power is absorbed, and the temperature of the absorber changes. This temperature change results in a change in the Seebeck voltage from the thermocouples, between the ends of the thermocouples connected to the frame and the ends of the thermocouples connected to the absorber. Since the thermocouples are connected in series, the voltage change across each thermocouple is added to the voltage of the remaining thermocouples.
Unfortunately, sensed changes in temperature result in relatively small changes in output voltage of such sensors. Accordingly, it is desirable to increase the output voltage to improve the resolution of the device. One way to increase the output voltage change due to a temperature change of a target object is by increasing the amount of infrared radiation received by the sensor by using a lens such as a refractive lens or a fresnel lens. Refractive lenses made of infrared transmitting materials, however, are typically expensive and must be carefully positioned. Fresnel lenses, which use diffraction, require a relatively large distance between the lens and the absorber, resulting in a larger sensor package. Alternatively, curved surfaces of revolution, or a Winston light concentrator in which a section of a parabola is revolved to form the reflecting surface, could be incorporated into the device. However, the manufacturing process for micromachined devices such as thermopile infrared detectors makes the incorporation of such shapes for each detector commercially undesirable.
The present invention provides an infrared sensor including an absorber for absorbing incident infrared power to produce a signal representing the temperature of a target object and a frame supporting the absorber which includes a plurality of reflecting surfaces disposed about the circumference of the absorber for reflecting incident infrared power toward the absorber to increase the output voltage resulting from a given change in temperature of the target object. In one embodiment of the invention, the reflection surfaces are formed on the etched sidewalls of the cavity which are created behind a membrane including the absorber. The cavity is formed during the standard fabrication process of the thermopile. As infrared power reaches the sensor, incident infrared power is reflected off of the reflecting surfaces toward the absorber, thereby increasing the difference in temperature between the absorber and the frame. The reflecting surfaces may be covered with a metal film to further increase the absorbed power of the sensor.
According to another embodiment of the invention, a separate rectangular frame structure is produced by micromachining silicon to form a light concentrator that may be attached to the front side of a thermopile sensor using appropriate adhesive material. The light concentrator includes reflecting surfaces which also reflect incident infrared power to the absorber. In this embodiment, metal may be deposited on the entire frame during a fabrication step already used to manufacture the thermopile detector.
These and other features of the present invention will become more apparent and the invention will be better understood upon consideration of the following description and the accompanying drawings.